The objective of this project was to develop a means of detecting and quantifying foam cells (i.e., cholesteryl ester-containing cells) in atherosclerotic vessels using flow cytometry and sorting. Single call suspensions of aortic cells were prepared by enzymatic dissociation of aortas from swine with spontaneous and experimentally induced (using idetary cholesterol) atherosclerosis. Cellular cholesteryl ester was enzymatically hydrolyzed and specifically stained with the fluorescent dye filipin. The numbers of cholesteryl ester-containing cells in each aorta were determined using flow cytometry. Animals maintained on a cholesterol-free diet showed an increasingly variable number of foam cells as these animals increased in age. It was of interest that the atherosclerotic process appears to begin at a very early age in swine, as even some six-month-old animals had small numbers of foam cells. Some older animals had elevated numbers of foam cells, whereas other older animals had numbers of foam cells similar to levels found in younger 6-month-old animals. There was no statistical correlation of foam cell number and serum cholesterol level in the spontaneous disease group. A possible genetic basis for the variability in atherosclerosis among these animals was found. Although elevation of serum cholesterol induced by cholesterol feeding was associated with higher foam cell number, the correlation of serum cholesterol and foam cell number was not strong. Reversal of cholesterol feeding rsulted in a reversal of the cholesterol-induced stimulation of foam cell formulation. We have shown that "foam cells" can be detected and quantified using flow cytometry. This highly sensitive assay provides a means to detect and quantify early atherosclerotic events. In addition, this assay may be useful in quantifying circulating "foam cells" providing a non-invasively derived index of the atherosclerotic process.